The Study On Antinociceptive Influence And Possible Mechanism Of Dexmedetomidine On Inflammatory Visceral Pain In Rats

Pain is one of the most common and serious problems in clinic,along with the occurrence and progression of diseases.Visceral pain,with characteristics of fuzzy positioning and often be accompanied by referred pain and physiological and psychological responses of patients,is difficult to diagnosis and treatment,is prone to result in pain sensitization.Although it was previously thought that visceral pain was a type of somatic pain,but proved by lots of evidences,it differs significantly in terms of neurological mechanisms.A variety of thoraco-abdominal and pelvic diseases,including tumors,cause patients to experience severe visceral pain.Minimally invasive diagnosis and treatment techniques,such as laparoscopy,thoracoscopy and endoscopy,are applied to clinical increasingly,injury originating from surgery incision is significantly reduced.However,the injury and noxious stimulation on the internal organs,are almost similar with open surgery in operation practiced by laparoscopy and thoracoscopy,and existence in endoscopic manipulating yet.In perioperation,patients still suffer from severe visceral pain deriving from inflammation,ischemia,traction and others,caused by the surgery practices and diseases.Currently,opioids play a dominant role in the treatment of severe acute pain during perioperative period,including acute visceral pain related to surgery and diseases.But,when used in large doses and or sensitive patients,opioids can lead to a number of dose-related side effects,such as vomiting and nausea,drowsiness,constipation,urinary retention,pruritus and respiratory depression,all mentioned can hinder patients’recovery and may give rise to serious complications.Agonists ofα₂-adrenoceptor,is widely used in anti-hypertensive,sedation and analgesia on critical and surgical patients.Dexmedetomidine（DEX）is a highly selectiveα₂-adrenoceptor agonist with the affinity ofα₂-adrenergic receptor is 8 times higher than that of clonidine,a classicalα₂-adrenoceptor agonist.Evidences showed,using of DEX during perioperative period can reduce the dosage of opioids and sedatives,effectively inhibit the stress response.However,so far,no study has investigated whether DEX can alleviate symptoms of inflammation-related visceral pain or explore its related mechanism.Nevertheless,by far,to our knowledge,there is no study to investigate whether DEX can alleviate symptoms of inflammation-related visceral pain or explore its related mechanism.Objective:The study is aimed to characterize the antinociceptive influence of DEX pretreated in epidural on acute inflammatory visceral pain（AIVP）induced by acetic acid in rats and to evaluate whether antinociception is regulated by the ERK pathway,Toll-like receptor signaling,and the transient receptor potential（TRP）channel.Methods:Thirty SD rats were randomly divided into six groups,with five rats in each group:control（rats received no treatment）,vehicle（rats were subdurally injected with0.25 ml 5%DMSO）,model group（rat received no treatment after model establishing）,low dose DEX group（lDEX;rats were subdurally injected with 5 ug DEX in 5%DMSO 0.25ml）,medium dose DEX group（mDEX;rats were subdurally injected with25 ug DEX in 5%DMSO 0.25ml）,high dose DEX group（hDEX;rats were subdurally injected with 50 ug DEX in 5%DMSO 0.25ml）.Fifteen minutes after the subdural injection of vehicle or DEX,300 mg/kg acetic acid（3%）was injected intraperitoneally,and we observed the rats for half an hour.Thermal withdrawal latency（TWL）,mechanical withdrawal threshold（MWT）,and abdominal withdrawal reflex（AWR）were used to assess pain behavior.We also assessed changes in mean arterial blood pressure（MAP）and electromyographic（EMG）response to colorectal distension.Interleukin 1（IL-1）,IL-2,and IL-6 in serum,and tumor necrosis factorα（TNF-α）and prostaglandin E₂（PGE₂）in peritoneal fluid were detected by ELISA.The expression levels of phospho（p）CREB,pERK1/2,and pMEK1,the activation state of Toll-like receptor 4（TLR4）,and transient receptor potential cation channel,subfamily V,member 1（TRPV1）channel expression in the spinal cord of rats were determined by real-time PCR and western blot.Results:AWR scores and amplitude of EMG were significantly elevated,scores of TWL and MWT were significantly decreased in the model group and vehicle group compared to that in the controls（P<0.05）.Compared to the model group,post-intervention AWR scores,amplitude of EMG were lower in the mDEX group and hDEX group（P<0.05）,scores of TWL and MWT were reduced too（P<0.05）.Pre-treatment administration in epidural of Dexmedetomidine,suppressed release of IL-1βand IL-6 in rats’serum,TNF-αand PGE₂ in peritoneal fluid,increased release of IL-2 in serum,the suppression,in medium and high doses of Dexmedetomidine,are more effective than that in low dose group（P<0.05）.AIVP from acetic acid and stimulation during AWR on rats,caused rise of rats’MAP,the MAP were lower after stimulation of AWR with 80mmHg in three group with Dexmedetomidine pre-treatment administration compared with model group and vehicle group,that were lower even in medium and high doses of Dexmedetomidine than that in control group（P<0.05）.Acetic acid,led to the activation of TLR4 and its downstream target,NF-κB and IRF3 in spinal dorsal horn.Converse,the mRNA levels and proteins’expression of TLR4,as well phosphorylation of NF-κB and IRF3,were suppressed by Dexmedetomidine（P<0.05）.In comparison to control group,the mRNA levels and proteins’expression of ERK,phosphorylation of ERK,and the key adjust proteins of MEK、CREB in pathway of ERK in same place in rats,are higher in model group and the increasing of mentioned,were suppressed by Dexmedetomidine（P<0.05）.Moreover,TRPV1 channel,along with CGRP in spinal dorsal horn,were activated by acetic acid,suppressed by Dexmedetomidine（P<0.05）.Conclusions:Taken together,our results suggest that DEX administration has an antinociceptive effect on AIVP in rats through the MEK/ERK pathway,TLR signaling,and the TRPV1 channel,suppressing inflammatory visceral hypersensitivity.